1. Field of the Invention
The present invention relates to a package structure preventing solder overflow on substrate solder pads, especially to a package structure using solder pad ridges or solder pad grooves to reduce solder overflow.
2. The Prior Art
The growing requirement for smaller electronic products has augmented the demand for flip-chip technology in system-in-package (SIP). However, the package process has faced a new problem, which is the occurrence of solder spread on a big solder pad. It makes the solder unable to bridge the solder joints after reflow, so that it affects the reliability of the package.
Referring to FIG. 1, a conventional flip chip package structure 1 includes a plurality of die pins 15, a plurality of solders 40, and a plurality of substrate solder pads 30. The die pins 15 are included in a die 10 and the die pins 15 are located under the die 10. The substrate solder pads 30 are included in a substrate 20. The substrate solder pads 30 are formed on an upper surface of the substrate 20 by copper plating or etching. The solders 40 connect the die pins 15 with the corresponding substrate solder pads 30.
When applying the flip chip process of prior art to SIP, some of the substrate solder pads 30 needs to have large areas to meet the requirements of grounding and having multiple joints at the same time. Therefore, it makes the solder 40 overflow easily on the larger substrate solder pad 30 when packaging, as shown in region A of FIG. 1. Because of overflow, the solder 40 can not connect the die pin 15 with the substrate solder pad 30. Thus, the die pin 15 disconnects from the substrate solder pad 30. It causes a serious problem of reliability of the package.
Right now, it usually uses solder mask dams to prevent the overflow of the solder 40 without affecting electric properties. However, for a flip chip design, the solder mask dams on the die pad are likely too thick, which makes the distance between the die 10 and the substrate 20 too small. It would directly influence the yield of molding or underfilling. If the thickness of the solder mask dams is decreased, the solder mask peeling is likely to happen. The reliability is affected.